This application claims priority to an application entitled xe2x80x9cA Method For Calculating Optimal Number of BTSs In A Wireless Network and Determining A Loading Factor Value Thereforxe2x80x9d filed in the Korean Industrial Property Office on Jun. 19, 1998 and assigned Serial No. 98-023184, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to a wireless network and more particularly, to a method for calculating an optimal number of base transceiver stations (BTSs) in a wireless network and for determining loading factor values therefor. Additionally, the method chooses the number of BTSs to satisfy both coverage requirements and an optimum number of subscribers of the wireless network.
2. Description of the Related Art
In a cellular mobile telecommunication system or wireless network, the whole service area is divided into a plurality of coverage areas, that is, cells, each of which includes a base transceiver station (BTS). The BTSs are controlled by a mobile switching center to enable subscribers to continue communicating while moving from one cell to another. BTSs are connected to the mobile switching center via a wire link and the mobile switching center is connected to another mobile switching center or a public switched telephone network (PSTN).
In a cellular mobile telecommunication system, a frequency of a mobile station is not fixed to a specific channel, but is automatically followed by a frequency which a respective BTS designates. Even though neighboring cells must use other frequencies, cells farther away can use the same frequency. Therefore, using this design configuration where a cellular mobile telecommunication system divides a service area into cells and reuses frequencies spatially, the system maintains a high operating efficiency in its usage of various frequencies and hence, it can accommodate many subscribers.
A mobile telecommunication system which uses a code division multiple access (CDMA) technique accommodates a plurality of subscribers on the same frequency by using codes. FIG. 1 is the well-known structure of the CDMA wireless network. In FIG. 1, base transceiver stations (BTSs) 10, 11, 12 and 13, base station controllers (BSCs) 20 and 21 and a mobile switching center 30 are included. Each BTS supports a cell, and the BSC couples a plurality of BTSs to the mobile switching center, which connects a plurality of BSCs to other mobile switching centers (MSC) or to a public switched telephone network (PSTN). Further a visitor location register (VLR) 31 is included which provides information about subscribers to the mobile switching center 30. The mobile switching center 30 uses information about subscribers to provide subscribers with service.
It is important to determine appropriate positions within a cell where a BTS should be placed and to calculate the total number of base transceiver stations in the overall cellular mobile telecommunication system to improve system performance and to minimize costs. The amount of subscribers to be serviced at a given time and the size of the coverage area should be taken into consideration in determining the positions and the total number of BTSs.
A wireless network typically calculates the number of subscribers which a base transceiver station can serve by multiplying a maximal traffic calculated theoretically with a loading factor value, where the loading factor value is the system accommodation limit or the acceptable capacity of the system, namely, the capacity of the load. The loading factor value, which is generally predetermined by the original equipment manufacturer, is generally chosen between 50% and 75%.
The loading factor value should be considered in determining the radius of a cell, since the loading factor value influences interference margin in a service distance analysis table. In other words, as the loading factor value increases, the number of subscribers that can be serviced by a BTS increases, i.e., traffic increases, but the cell radius, i.e., service coverage area, decreases.
In order to calculate acceptable service coverage area of one base station, the interference margin must be considered which provides a maximum allowable path loss (MAPL) between subscribers in cells. The interference margin is obtained by Equation 1:
Interference Margin=10xc3x97log(1xe2x88x92Loading Factor).xe2x80x83xe2x80x83Equation 1
According to this equation, the greater the value of MAPL, the base station is to service a greater service coverage area. Therefore, the service coverage area of one base station is proportional to the maximum MAPL of the base station. The MAPL is in reverse proportion to the loading factor, and hence, the service coverage area of one base station is reverse proportional to the loading factor. Namely, as the loading factor increases, the service coverage area of one base station decreases and the number of BTSs required to cover the whole service coverage area increases.
Accordingly, it is important to take into consideration the loading factor value in the design of a wireless network. In the wireless network, the number of BTSs is given by Equation 2.
The number of BTSs=Max [(the number of BTSs given by the traffic), (the number of BTSs given by service coverage)]xe2x80x83xe2x80x83Equation 2
This equation calculates the optimal numbers of BTSs by using the same loading factor value regardless of geographical features, such as the distribution of subscribers in a coverage area. As a result, both the traffic and coverage size are not taken into consideration which are required in determining the loading factor value.
One example of calculating the number of BTSs in a wireless network according to the prior art will now be described. A region A has 100,000 subscribers within a coverage size of 3,000 km2. It is assumed that in terms of morphology, the region A is composed of 10% dense urban district, 20% urban district, 20% suburban district and 50% rural district. Also, it is assumed that 70% of the subscribers exist in a sector-cell and the rest, i.e., 30%, exist in an omni-cell. When a loading factor value is fixed at 50%, 262 BTSs are needed to accommodate the given the amount of subscribers (traffic), but only 88 BTSs are needed to accommodate the given coverage size. Therefore, 262 BTSs are determined as needed, since 262 is greater than 88. Accordingly, in this example, the optimal number of BTSs is determined by the number of subscribers or traffic.
However, a region which has a little traffic is influenced more by the coverage size than by the number of subscribers as the following prior art example illustrates. A region B has 250,000 subscribers within a coverage size of 4,000 km2 and has the same morphological make up and distribution of subscribers as the first example. When a loading factor value is fixed as 50%, 66 BTSs are needed to accommodate the given amount of subscribers, but 118 BTSs are needed to accommodate the given coverage size. Therefore, since 118 is greater than 66, the number of BTSs needed is 118. Accordingly, in this second example, the optimal number of BTSs is determined by the coverage size.
As shown by the two examples, there is a big difference between the number of BTSs which accommodates the given traffic conditions and the number of BTSs which accommodates the given coverage size. The examples illustrate that it is inappropriate to apply a loading factor value according to the geographical features of the coverage area. Because the prior art uniformly uses a loading factor value regardless of the geographical features, it is impossible to calculate the number of BTSs to accommodate both the given traffic conditions and the given coverage size.
It is, therefore, an object of the present invention to provide a method for calculating an optimal number of BTSs by taking into consideration the given traffic conditions and given coverage area size.
It is another object of the present invention to provide a method for properly determining a loading factor value based on geographical features for calculating the optimal number of BTSs.
Other objects and advantages of the present invention will become apparent with reference to the following detailed description and the attached drawings.
The present invention provides, in a wireless network having a plurality of base transceiver stations providing communication service to mobile stations in a given coverage area, a method for calculating an optimal number of base transceiver stations (BTSs). The method includes the steps of calculating a first number of BTSs in accordance with a decrease in traffic as a loading factor value increases, wherein the loading factor value indicates an accommodation limit of each of the BTSs; calculating a second number of BTSs as given by an increase in the coverage area or size as the loading factor value increases; determining a loading factor value which minimizes a difference between the first and second numbers; and determining the optimal number of BTSs based on the determined loading factor value.